- License: Remove "CC-BY-SA-2.0" as explicit license type, but make note about unclear provenance of some images

- Docs: Update license badges to reflect lack of (known) CC-BY-SA-2.0 usage
- Replace CC-BY-SA-2.0 polygon image (thanks, @mowijo !)

README.md

@@ -18,9 +18,12 @@
 
 (see also [licenses for dev. deps.](https://raw.githubusercontent.com/SVG-Edit/svgedit/master/badges/licenses-badge-dev.svg?sanitize=true))
 
-(Note: The protective license `CC-BY-SA-2.0` is due to our containing at least
-one image under this stricter license; to avoid its conditions, we'd
-appreciate a replacement image. See [#377](https://github.com/SVG-Edit/svgedit/issues/377))
+(Note: The license provenance of the images in `/editor/images` may not be
+fully clear, even with the origin of some of the images listed as being from <http://tango.freedesktop.org/static/cvs/tango-art-libre/22x22/>. We would like to
+replace these images if their provenance cannot be determined or is found to
+be under a protective license. If you know of the original terms, or can help
+create SVG replacement images, please let us know at:
+[#377](https://github.com/SVG-Edit/svgedit/issues/377).)
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